This invention relates to a control circuit for a forced-air heating system.
A conventional domestic forced-air central heating system comprises a furnace in which fuel is burned to generate heat, a plenum defining an interior space which is in heat exchange relationship with the furnace and has an inlet chamber and an outlet chamber, ducting in open communication with the inlet chamber and the outlet chamber, and an electrically-driven blower for creating a flow of air through the plenum and the ducting. A room thermostat senses the temperature existing in a living space served by the heating system, and if the sensed temperature falls below a set temperature the room thermostat issues a command to the furnace to cause fuel to be burned. The motor that drives the blower is controlled by a temperature sensor in the plenum. The plenum temperature sensor may employ a bimetallic strip. If the sensed plenum temperature exceeds a set value, the blower motor is energized; otherwise it is not energized. Typically, the blower motor is energized when the plenum temperature reaches about 57.degree. C.
It is conventional for a room thermostat to have a so-called night-time set-back, such that the temperature in the living space is permitted at night to fall below the normal daytime occupation temperature. When the room thermostat first calls for heat in the morning, the air in the ducting is cold. Therefore, when the blower motor is turned on, a current of cold air is driven into the living space and this may cause discomfort to occupants of the living space.
The furnace of a domestic forced-air central heating system may employ gas or oil as the fuel. When the room thermostat of a gas or oil fired central heating system calls for additional heat, a control valve for supplying fuel to the furnace is opened and the fuel is ignited, e.g. by a pilot flame. Another fuel that is used in domestic central heating systems is wood pellets. A wood pellet furnace employs a feed mechanism, such as an auger, for feeding pellets to a fire grate and a draft-inducing fan for maintaining a current of air through the fire grate to support combustion. Normally, operation of the pellet feed mechanism and the draft inducer are controlled by the room thermostat. When the room thermostat does not call for heat, the draft inducer is turned off and the pellet feed mechanism delivers fuel at a very low rate, sufficient only to maintain a pilot fuel in the grate. There is a danger that the pilot fire will be extinguished. However, the feed mechanism will continue to deliver pellets, with the result that the grate will be filled with pellets and the furnace cannot be operated again without removing almost all the pellets and relighting the pilot fire.